Scientists Identify Key Smell that Attracts Mosquitoes to Humans

Chemical ecologists in the Walter Leal lab at the University of California, Davis, have identified the dominant odor naturally produced in humans and birds that attracts the blood-feeding Culex mosquitoes, which transmits West Nile virus and other life-threatening diseases.

The groundbreaking research explains why mosquitoes shifted hosts from birds to humans and paves the way for key developments in mosquito and disease control.

Entomology professor Walter Leal and postdoctoral researcher Zain Syed, UC Davis Department of Entomology, found that nonanal (pronounced "no-nan-al") is the powerful semiochemical that triggers the mosquitoes’ keen sense of smell, directing them toward a blood meal. A semiochemical is a chemical substance or mixture that carries a message.

“Nonanal is how they find us,” Leal said. “The antennae of the Culex quinquefasciatus are highly developed to detect even extremely low concentrations of nonanal.” Mosquitoes detect smells with the olfactory receptor neurons of their antennae.

Yale University professor John Carlson, a leading scientist in insect olfaction, described the study as “exciting with important implications for the intriguing question of how mosquitoes find the humans they bite.”

“Leal and Syed have identified a human odor that is detected with great sensitivity by the antennae of mosquitoes that transmit West Nile virus,” Carlson said. “In addition to its scientific interest, the study may have important practical applications in the control of these mosquitoes and the diseases they carry.”

Birds, the main hosts of mosquitoes, serve as the reservoir for the West Nile virus, Leal said. When infected mosquitoes take a blood meal, they transmit the virus to their hosts, including birds, humans, horses, dogs, cats, bats, chipmunks, skunks, squirrels and domestic rabbits.  Since 1999, the Centers for Disease Control and Prevention has recorded 29,397 human cases and 1,147 fatalities in the United States alone.

The UC Davis researchers tested hundreds of naturally occurring compounds emitted by multiracial and multiethnic humans and multiple species of birds, including chickens and pigeons. They collected chemical odors from 16 human subjects, including two blacks, two Chinese, eight Caucasians, three Latinos and one East Indian, ranging in age from 20 to 55.  “We then determined the specificity and sensitivity of the olfactory receptor neurons (ORNS) to the isolated compounds on the antennae of the mosquitoes,” Syed said.

Their newly published research, titled “Acute Olfactory Response of Culex Mosquitoes to a Human- and Bird-Derived Attractant,” appears in the current edition of Proceedings of the National Academy of Sciences. (Download research paper: PDF)

Leal and Syed found that nonanal acts synergistically with carbon dioxide, a known mosquito attractant.  “We baited mosquito traps with a combination of nonanal and carbon dioxide and we were drawing in as many as 2000 a night in Yolo County, near Davis,” Syed said. “Nonanal, in combination with carbon dioxide, increased trap captures by more than 50 percent, compared to traps baited with carbon dioxide alone.”

“This study highlights the importance of basic research not only to the advancement of science but also public health,” said Laura Kramer, director of the Arbovirus Laboratories, Wadsworth Center, New York State Department of Health, and a professor in the School of Public Health, State University of New York. “The findings provide a mechanistic basis for host switching from birds to humans by mosquitoes feeding in late summer. The results are enticing as a tool that may be applied to mosquito control efforts, especially if the same semiochemical proves equally effective in attracting other important Culexvectors such as Culex pipiens and Culex tarsalis, and even more significantly, Aedes aegypti and Aedes albopictus, vectors of dengue and Chikungunya virus.”